Exercise aortic stiffness: reproducibility and relation to end-organ damage in men.

Resting aortic stiffness (pulse wave velocity; aortic PWV (aPWV)) independently predicts end-organ damage and mortality. Exercise haemodynamics have been shown to unmask cardiovascular abnormalities, otherwise undetectable at rest, but the response of aPWV to exercise has never been examined. This study aimed to develop a technique to measure exercise aPWV, determine reproducibility and relation to subclinical end-organ damage with aging. Healthy younger (n=17, 30±8 years) and older (n=18, 54±8 years) untreated men underwent cardiovascular assessment at rest and during low intensity semirecumbent cycling. Tonometry was used to assess aPWV and central blood pressure (BP). All participants underwent 24 h ambulatory BP (ABP) monitoring. Kidney function was assessed by estimated glomerular filtration rate (eGFR). Fifteen participants had testing repeated within 28±18 days. Exercise aPWV had good reproducibility (mean difference=-0.35±0.61 m s(-1), intraclass correlations=0.874, P<0.001) and was increased 26% above resting values in younger men (5.8±0.9 vs 7.3±1.6 m s(-1), P<0.001) and 19% above resting values in older men (6.3±1.0 vs 7.4±0.9 m s(-1), P<0.001). Exercise, but not resting, aPWV was significantly correlated with eGFR in older men (r=-0.633, P=0.005), and this was maintained after correction for age, body mass index and daytime systolic ABP (r=-0.656, P=0.008). Conversely, in younger men there was no significant association between eGFR and aPWV either at rest (r=-0.031, P=0.906) or during exercise (r=-0.117, P=0.655). Exercise aPWV is reproducible and significantly associated with kidney function in healthy older men. Further studies to determine the physiology and clinical relevance of raised exercise aPWV are warranted.

Microvascular blood flow responses to muscle contraction are not altered by high-fat feeding in rats.

AIM: Exercise and insulin each increase microvascular blood flow and enhance glucose disposal in skeletal muscle. We have reported that insulin-mediated microvascular recruitment in a diet-induced model of insulin resistance (high-fat feeding for 4 weeks) is markedly impaired; however, the effect of muscle contraction in this model has not been previously explored. METHODS: We fed rats either normal (ND, 10% calories from fat) or high-fat (HFD, 60% calories from fat) diets ad libitum for 4-8 weeks. Animals were then anaesthetized and one hindlimb electrically stimulated to contract at 0.05, 0.1 and 2 Hz (field stimulation, 30 V, 0.1 ms duration) in 15 min stepwise increments. Femoral artery blood flow (Transonic flow probe), muscle microvascular blood flow (hindleg metabolism of 1-methylxanthine and contrast-enhanced ultrasound) and muscle glucose disposal (uptake of radiolabelled 2-deoxy-d-glucose and hindleg glucose disappearance) were measured. RESULTS: Both ND and HFD rats received the same voltage across the leg and consequently developed the same muscle tension. Femoral artery blood flow in the contracting leg increased during 2 Hz contraction, but not during the lower frequencies and these effects were similar between ND and HFD rats. Muscle microvascular blood flow significantly increased in a contraction frequency-dependent manner, and preceded increases in total limb blood flow and these effects were similar between ND and HFD rats. Muscle glucose disposal was markedly elevated during 2 Hz contraction and was comparable between ND and HFD rats. CONCLUSION: Contraction-mediated muscle microvascular recruitment and glucose uptake are not impaired in the HFD insulin resistant rat.

cGMP phosphodiesterase inhibition improves the vascular and metabolic actions of insulin in skeletal muscle.

There is considerable support for the concept that insulin-mediated increases in microvascular blood flow to muscle impact significantly on muscle glucose uptake. Since the microvascular blood flow increases with insulin have been shown to be nitric oxide-dependent inhibition of cGMP-degrading phosphodiesterases (cGMP PDEs) is predicted to enhance insulin-mediated increases in microvascular perfusion and muscle glucose uptake. Therefore, we studied the effects of the pan-cGMP PDE inhibitor zaprinast on the metabolic and vascular actions of insulin in muscle. Hyperinsulinemic euglycemic clamps (3 mU·min(-1)·kg(-1)) were performed in anesthetized rats and changes in microvascular blood flow assessed from rates of 1-methylxanthine metabolism across the muscle bed by capillary xanthine oxidase in response to insulin and zaprinast. We also characterized cGMP PDE isoform expression in muscle by real-time PCR and immunostaining of frozen muscle sections. Zaprinast enhanced insulin-mediated microvascular perfusion by 29% and muscle glucose uptake by 89%, while whole body glucose infusion rate during insulin infusion was increased by 33% at 2 h. PDE2, -9, and -10 were the major isoforms expressed at the mRNA level in muscle, while PDE1B, -9A, -10A, and -11A proteins were expressed in blood vessels. Acute administration of the cGMP PDE inhibitor zaprinast enhances muscle microvascular blood flow and glucose uptake response to insulin. The expression of a number of cGMP PDE isoforms in skeletal muscle suggests that targeting specific cGMP PDE isoforms may provide a promising avenue for development of a novel class of therapeutics for enhancing muscle insulin sensitivity.

Effects of central administration of insulin or l-NMMA on rat skeletal muscle microvascular perfusion.

AIM: Intracerebroventricular (ICV) administration of a nitric oxide synthase (NOS) inhibitor to rats has been reported to raise blood pressure (BP) and cause insulin resistance, suggestive of a central effect of insulin that is NO dependent. Herein we test whether ICV insulin has peripheral haemodynamic and metabolic effects and whether peripheral effects of systemic insulin are affected by the ICV administration of the NOS inhibitor N(G) -methyl-l-arginine (l-NMMA). METHODS: Anaesthetized rats were fitted with an ICV cannula for insulin, artificial cerebrospinal fluid (aCSF) or l-NMMA infusion. Rats receiving ICV l-NMMA (500 µg) underwent systemic insulin clamp (10 mU/min/kg) or saline treatment for 70 min and were compared with animals receiving an equal amount of l-NMMA infused systemically. RESULTS: ICV aCSF or insulin (135 mU/min/kg brain) for 70 min or systemic l-NMMA (500 µg) had no effect on BP, heart rate (HR), femoral blood flow (FBF), glucose infusion rate, muscle 2-deoxyglucose uptake, microvascular perfusion or plasma insulin. However, ICV l-NMMA reduced systemic insulin-mediated increases in FBF (2.05 ± 0.08 to 1.55 ± 0.15 ml/min), 2-deoxyglucose uptake (17.7 ± 0.15 to 10.0 ± 0.03 µg/g/min) and microvascular perfusion (10.5 ± 0.5 to 6.6 ± 1.1 mol/min) (each mean ± SE, p < 0.05); plasma insulin, HR and BP were unaffected. CONCLUSIONS: Central insulin administration had no effect on skeletal muscle haemodynamics or glucose metabolism. However, systemic insulin-mediated increases in limb blood flow, muscle microvascular perfusion and glucose uptake may be regulated by a central pathway that is NO dependent.

Loss of insulin-mediated microvascular perfusion in skeletal muscle is associated with the development of insulin resistance.

AIM: The aetiology of the development of type 2 diabetes remains unresolved. In the present study, we assessed whether an impairment of insulin-mediated microvascular perfusion occurs early in the onset of insulin resistance. MATERIALS AND METHODS: Hooded Wistar rats were fed either a normal diet (ND) or a high-fat diet (HFD) for 4 weeks. Anaesthetized animals were subjected to an isoglycaemic hyperinsulinaemic clamp (3 or 10 mU/min/kg x 2 h), and measurements were made of glucose infusion rate (GIR), hindleg glucose uptake, muscle glucose uptake by 2-deoxy-d-glucose (R'g), glucose appearance (Ra), glucose disappearance (Rd), femoral blood flow (FBF) and hindleg 1-methylxanthine disappearance (1-MXD, an index of microvascular perfusion). RESULTS: Compared with ND-fed animal, HFD feeding led to a mild increase in fasting plasma glucose and plasma insulin, without an increase in total body weight. During the clamps, HFD rats showed an impairment of insulin-mediated action on GIR, hindleg glucose uptake, R'g, Ra, Rd and FBF, with a greater loss of insulin responsiveness at 3 mU/min/kg than at 10 mU/min/kg. The HFD also impaired insulin-mediated microvascular perfusion as assessed by 1-MXD. Interestingly, 1-MXD was the only measurement that remained unresponsive to the higher dose of 10 mU/min/kg insulin. CONCLUSIONS: We conclude that the early stage of insulin resistance is characterized by an impairment of the insulin-mediated microvascular responses in skeletal muscle. This is likely to cause greater whole body insulin resistance by limiting the delivery of hormones and nutrients to muscle.

Local hindlimb antioxidant infusion does not affect muscle glucose uptake during in situ contractions in rat.

There is evidence that reactive oxygen species (ROS) contribute to the regulation of skeletal muscle glucose uptake during highly fatiguing ex vivo contraction conditions via AMP-activated protein kinase (AMPK). In this study we investigated the role of ROS in the regulation of glucose uptake and AMPK signaling during low-moderate intensity in situ hindlimb muscle contractions in rats, which is a more physiological protocol and preparation. Male hooded Wistar rats were anesthetized, and then N-acetylcysteine (NAC) was infused into the epigastric artery (125 mg.kg(-1).h(-1)) of one hindlimb (contracted leg) for 15 min before this leg was electrically stimulated (0.1-ms impulse at 2 Hz and 35 V) to contract at a low-moderate intensity for 15 min. The contralateral leg did not receive stimulation or local NAC infusion (rest leg). NAC infusion increased (P<0.05) plasma cysteine and cystine (by approximately 360- and 1.4-fold, respectively) and muscle cysteine (by 1.5-fold, P=0.001). Although contraction did not significantly alter muscle tyrosine nitration, reduced (GSH) or oxidized glutathione (GSSG) content, S-glutathionylation of protein bands at approximately 250 and 150 kDa was increased (P<0.05) approximately 1.7-fold by contraction, and this increase was prevented by NAC. Contraction increased (P<0.05) skeletal muscle glucose uptake 20-fold, AMPK phosphorylation 6-fold, ACCbeta phosphorylation 10-fold, and p38 MAPK phosphorylation 60-fold, and the muscle fatigued by approximately 30% during contraction and NAC infusion had no significant effect on any of these responses. This was despite NAC preventing increases in S-glutathionylation with contraction. In conclusion, unlike during highly fatiguing ex vivo contractions, local NAC infusion during in situ low-moderate intensity hindlimb contractions in rats, a more physiological preparation, does not attenuate increases in skeletal muscle glucose uptake or AMPK signaling.

Acute blockade by endothelin-1 of haemodynamic insulin action in rats.

AIMS/HYPOTHESIS: Plasma levels of endothelin-1 are frequently elevated in patients with hypertension, obesity and type 2 diabetes. We hypothesise that this vasoconstrictor may prevent full perfusion of muscle, thereby limiting delivery of insulin and glucose and contributing to insulin resistance. MATERIALS AND METHODS: The acute effects of endothelin-1 on insulin-mediated haemodynamic and metabolic effects were examined in rats in vivo. Endothelin-1 (50 pmol min(-1) kg(-1) for 2.5 h) was infused alone, or 30 min prior to a hyperinsulinaemic-euglycaemic insulin clamp (10 mU min(-1) kg(-1) for 2 h). Insulin clamps (10 or 15 mU min(-1) kg(-1)) were performed after 30 min of saline infusion. RESULTS: Endothelin-1 infusion alone increased plasma endothelin-1 11-fold (p < 0.05) and blood pressure by 20% (p < 0.05). Endothelin-1 alone had no effect on femoral blood flow, capillary recruitment or glucose uptake, but endothelin-1 with 10 mU min(-1) kg(-1) insulin caused a decrease in insulin clearance from 0.35 +/- 0.6 to 0.19 +/- 0.02 ml/min (p = 0.02), resulting in significantly higher plasma insulin levels (10 mU min(-1) kg(-1) insulin: 2,120 +/- 190 pmol/l; endothelin-1 + 10 mU min(-1)kg(-1) insulin: 4,740 +/- 910 pmol/l), equivalent to 15 mU min(-1) kg(-1) insulin alone (4,920 +/- 190 pmol/l). The stimulatory effects of equivalent doses of insulin on femoral blood flow, capillary recruitment and glucose uptake were blocked by endothelin-1. CONCLUSIONS/INTERPRETATION: Endothelin-1 blocks insulin's haemodynamic effects, particularly capillary recruitment, and is associated with decreased muscle glucose uptake and glucose infusion rate. These findings suggest that elevated endothelin-1 levels may contribute to insulin resistance of muscle by increasing vascular resistance and limiting insulin and glucose delivery.

Vascular and metabolic effects of methacholine in relation to insulin action in muscle.

AIMS/HYPOTHESIS: Methacholine (MC) is a nitric oxide vasodilator, but unlike other vasodilators, it potentiates insulin-mediated glucose uptake by muscle. The present study aimed to resolve whether this action was the result of a vascular effect of MC leading to increased muscle perfusion or a direct effect of MC on the myocytes. We hypothesise that vascular-mediated insulin-stimulated glucose uptake responses to MC occur at lower doses than direct myocyte MC-mediated increases in glucose uptake. METHODS: The vascular and metabolic effects of this vasodilator were examined in rats in vivo using a novel local infusion technique, and in the pump-perfused rat hindlimb under conditions of constant flow. RESULTS: Local infusion of low-dose MC (0.3 micromol/l) into the epigastric artery of one leg (test) in vivo markedly increased femoral blood flow and decreased vascular resistance, without effects in the contra-lateral leg. Capillary recruitment, but not glucose uptake, was increased in the test leg. All increases caused by MC were confined to the test leg and blocked by local infusion into the test leg of N-nitro-L-arginine methyl ester (L-NAME), but not by infusion of N-nitro-D-arginine methyl ester (D-NAME). In the constant-flow pump-perfused rat hindlimb, infusion of 0.6 micromol/l MC vasodilated the pre-constriction effected by 70 nmol/l noradrenaline or 300 nmol/l serotonin, and this was blocked by 10 micromol/l L-NAME. 2-Deoxyglucose in muscle was increased by 30 micromol/l MC (p<0.05), but was unaffected by 3 micromol/l MC. All increases in 2-deoxyglucose uptake by 30 micromol/l MC were blocked by 10 micromol/l L-NAME. CONCLUSIONS/INTERPRETATION: MC has dose-dependent effects both on the vasculature and on muscle metabolism. At low dose (0.3-3 micromol/l), MC is a potent vasodilator in muscle, both in vivo and in vitro, without metabolic effects; at higher doses (> or =30 micromol/l) MC has a direct metabolic effect leading to increased glucose uptake. Both the vascular and metabolic effects are sensitive to L-NAME. The low-dose enhancement of insulin action in vivo by MC, which has been reported previously, thus seems to be attributable to vascular effects.

Endothelial Na+-D-glucose cotransporter: no role in insulin-mediated glucose uptake.

A recent report indicates that the Na+-D-glucose cotransporter SGLT1 is present in capillaries of skeletal muscle and is required for insulin-mediated glucose uptake in myocytes. This result is based on the complete inhibition of insulin-mediated muscle glucose uptake by phlorizin, an inhibitor of SGLT1. Using the pump-perfused rat hind limb, we measured glucose uptake, lactate efflux, and radioactive 2-deoxyglucose uptake into individual muscles with saline (control), phlorizin, insulin, and insulin plus phlorizin, as well as with saline and insulin using normal and low Na+ perfusion buffer. Insulin-mediated glucose uptake was not inhibited after correction for phlorizin interference in the glucose assay. Lactate efflux and 2-deoxyglucose uptake by individual muscles were unaffected by phlorizin. Low Na+ buffer did not affect insulin-mediated glucose uptake, lactate efflux, or 2-deoxyglucose uptake. We conclude that endothelial SGLT1 exerts no barrier for glucose delivery to myocytes.

Acute glucosamine-induced insulin resistance in muscle in vivo is associated with impaired capillary recruitment.

AIMS/HYPOTHESIS: Glucose toxicity and glucosamine-induced insulin resistance have been attributed to products of glucosamine metabolism. In addition, endothelial cell nitric oxide synthase is inhibited by glucosamine. Since insulin has endothelial nitric-oxide-dependent vasodilatory effects in muscle, we hypothesise that glucosamine-induced insulin resistance in muscle in vivo is associated with impaired vascular responses including capillary recruitment. MATERIALS AND METHODS: Glucosamine (6.48 mg kg(-1) min(-1) for 3 h) was infused with or without insulin (10 mU kg(-1) min(-1)) into anaesthetised rats under euglycaemic conditions. RESULTS: Glucosamine infusion alone increased blood glucosamine (1.9+/-0.1 mmol/l) and glucose (5.4+/-0.2 to 7.7+/-0.3 mmol/l) (p<0.05) but not insulin. Glucosamine induced both hepatic and muscle insulin resistance as evident from measures of glucose appearance and disposal as well as hind-leg glucose uptake, which was inhibited by approx. 50% (p<0.05). Insulin-mediated increases in femoral arterial blood flow and capillary recruitment were completely blocked by glucosamine. CONCLUSION/INTERPRETATION: Glucosamine mediates a major impairment of insulin action in muscle vasculature associated with the insulin resistance of muscle. Further studies will be required to assess whether the impaired capillary recruitment contributes to insulin resistance.

Local methacholine but not bradykinin potentiates insulin-mediated glucose uptake in muscle in vivo by augmenting capillary recruitment.

AIMS/HYPOTHESIS: Insulin has nitric-oxide-dependent vasodilatory effects in muscle, including capillary recruitment, that enhance access for itself and glucose. However, nitric-oxide-dependent vasodilators other than methacholine do not enhance insulin action. Our hypothesis is that methacholine, unlike bradykinin, enhances insulin-mediated glucose uptake in muscle by augmenting capillary recruitment. METHODS: Local infusion of either methacholine or bradykinin into one leg of the anaesthetised rat was made during physiological insulin (3 mU.kg(-1).min(-1)) infusion under euglycaemic conditions and without affecting systemic blood pressure. Whole-body glucose infusion was determined, as was femoral blood flow, 2-deoxyglucose uptake into calf muscles and the metabolism of infused 1-methylxanthine, a measure of capillary recruitment for each leg. RESULTS: Methacholine alone (0.3 micromol.l(-1)) increased femoral arterial blood flow, increased capillary recruitment but had no effect on 2-deoxyglucose uptake of the test leg relative to the contra-lateral control leg. Insulin alone (systemically) required a glucose infusion rate of 8.7 mg.kg(-1).min(-1) to maintain euglycaemia, increased 2-deoxyglucose uptake and capillary recruitment, but was without effect on femoral blood flow in either leg. Local methacholine with systemic insulin infusion increased femoral blood flow, 2-deoxyglucose uptake and capillary recruitment in the test leg only. Bradykinin (0.07 micromol.l(-1)), alone or with insulin, administered in a manner that increased femoral blood flow similarly to methacholine, did not affect 2-deoxyglucose uptake or capillary recruitment. CONCLUSIONS/INTERPRETATION: Methacholine but not bradykinin enhances insulin-mediated muscle glucose uptake in vivo. We conclude that methacholine acts at specific sites in the vasculature of muscle to stimulate capillary recruitment and thereby enhance insulin access.

Inhibiting NOS blocks microvascular recruitment and blunts muscle glucose uptake in response to insulin.

We examined the effects of inhibiting nitric oxide synthase with Nomega-nitro-l-arginine-methyl ester (l-NAME) on total hindlimb blood flow, muscle microvascular recruitment, and hindlimb glucose uptake during euglycemic hyperinsulinemia in vivo in the rat. We used two independent methods to measure microvascular perfusion. In one group of animals, microvascular recruitment was measured using the metabolism of exogenously infused 1-methylxanthine (1-MX), and in a second group contrast-enhanced ultrasound (CEU) was used. Limb glucose uptake was measured by arterial-venous concentration differences after 2 h of insulin infusion. Saline alone did not alter femoral artery flow, glucose uptake, or 1-MX metabolism. Insulin (10 mU.min-1.kg-1) significantly increased hindlimb total blood flow (0.69 +/- 0.02 to 1.22 +/- 0.11 ml/min, P < 0.05), glucose uptake (0.27 +/- 0.05 to 0.95 +/- 0.08 micromol/min, P < 0.05), 1-MX uptake (5.0 +/- 0.5 to 8.5 +/- 1.0 nmol/min, P < 0.05), and skeletal muscle microvascular volume measured by CEU (10.0 +/- 1.6 to 15.0 +/- 1.2 video intensity units, P < 0.05). Addition of l-NAME to insulin completely blocked the effect of insulin on both total limb flow and microvascular recruitment (measured using either 1-MX or CEU) and blunted glucose uptake by 40% (P < 0.05). We conclude that insulin specifically recruits flow to the microvasculture in skeletal muscle via a nitric oxide-dependent pathway and that this may be important to insulin's overall action to regulate glucose disposal.

Nonnutritive flow impairs uptake of fatty acid by white muscles of the perfused rat hindlimb.

Triglyceride hydrolysis by the perfused rat hindlimb is enhanced with serotonin-induced nonnutritive flow (NNF) and may be due to the presence of nonnutritive route-associated connective tissue fat cells. Here, we assess whether NNF influences muscle uptake of 0.55 mM palmitate in the perfused hindlimb. Comparisons were made with insulin-mediated glucose uptake. NNF induced during 60 nM insulin infusion inhibited hindlimb oxygen uptake from 22.0 +/- 0.5 to 9.7 +/- 0.8 micromol x g(-1) x h(-1) (P < 0.001), 1-methylxanthine metabolism (indicator of nutritive flow) from 5.8 +/- 0.4 to 3.8 +/- 0.4 nmol x min(-1) x g(-1) (P = 0.004), glucose uptake from 29.2 +/- 1.7 to 23.1 +/- 1.8 micromol x g(-1) x h(-1) (P = 0.005) and muscle 2-deoxyglucose uptake from 82.1 +/- 4.6 to 41.6 +/- 6.7 micromol x g(-1) x h(-1) (P < 0.001). Palmitate uptake, unaffected by insulin alone, was inhibited by NNF in extensor digitorum longus, white gastrocnemius, and tibialis anterior muscles; average inhibition was from 13.9 +/- 1.2 to 6.9 +/- 1.4 micromol x g(-1) x h(-1) (P = 0.02). Thus NNF impairs both fatty acid and glucose uptake by muscle by restricting flow to myocytes but, as shown previously, favors triglyceride hydrolysis and uptake into nearby connective tissue fat cells. The findings have implications for lipid partitioning in limb muscles between myocytes and attendant adipocytes.

Skeletal muscle microvascular recruitment by physiological hyperinsulinemia precedes increases in total blood flow.

Supraphysiological doses of insulin enhance total limb blood flow and recruit capillaries in skeletal muscle. Whether these processes change in response to physiological hyperinsulinemia is uncertain. To examine this, we infused either saline (n = 6) or insulin (euglycemic clamp, 3.0 mU x min(-1) x kg(-1), n = 9) into anesthetized rats for 120 min. Femoral artery flow was monitored continuously using a Doppler flow probe, and muscle microvascular recruitment was assessed by metabolism of infused 1-methylxanthine (1-MX) and by contrast-enhanced ultrasound (CEU). Insulin infusion raised plasma insulin concentrations by approximately 10-fold. Compared with saline, physiological hyperinsulinemia increased femoral artery flow (1.02 +/- 0.10 vs. 0.68 +/- 0.09 ml/min; P < 0.05), microvascular recruitment (measured by 1-MX metabolism [6.6 +/- 0.5 vs. 4.5 +/- 0.48 nmol/min; P < 0.05] as well as by CEU [167.0 +/- 39.8 vs. 28.2 +/- 13.8%; P < 0.01]), and microvascular flow velocity (beta, 0.14 +/- 0.02 vs. 0.09 +/- 0.02 s(-1)). Subsequently, we studied the time dependency of insulin's vascular action in a second group (n = 5) of animals. Using CEU, microvascular volume was measured at 0, 30, and 90 min of insulin infusion. Insulin augmented microvascular perfusion within 30 min (52.8 +/- 14.8%), and this persisted at 90 min (64.6 +/- 9.9%). Microvascular recruitment occurred without changes to femoral artery flow or beta. We conclude that insulin increases tissue perfusion by recruiting microvascular beds, and at physiological concentrations this precedes increases in total muscle blood flow by 60-90 min.

Exercise training improves insulin-mediated capillary recruitment in association with glucose uptake in rat hindlimb.

Exercise training is considered to be beneficial in the treatment and prevention of insulin insensitivity, and much of the effect occurs in muscle. We have recently shown that capillary recruitment by insulin in vivo is associated with and may facilitate insulin action to increase muscle glucose uptake. In the present study, we examined the effect of 14 days of voluntary exercise training on euglycemic-hyperinsulinemic clamped (10 mU. min(-1). kg(-1) for 2 h), anesthetized rats. Whole-body glucose infusion rate (GIR), hindleg glucose uptake, femoral blood flow (FBF), vascular resistance, and capillary recruitment, as measured by metabolism of infused 1-methylxanthine (1-MX), were assessed. In sedentary animals, insulin caused a significant (P < 0.05) increase in FBF (1.6-fold) and capillary recruitment (1.7-fold) but a significant decrease in vascular resistance. In addition, hindleg glucose uptake was increased (4.3-fold). Exercise training increased insulin-mediated GIR (24%), hindleg glucose uptake (93%), and capillary recruitment (62%) relative to sedentary animals. Neither capillary density nor total xanthine-oxidase activity in skeletal muscle were increased as a result of the training regimen used. We concluded that exercise training improves insulin-mediated increases in capillary recruitment in combination with augmented muscle glucose uptake. Increased insulin-mediated glucose uptake may in part result from the improved hemodynamic control attributable to exercise training.

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment.

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.

Nutritive blood flow affects microdialysis O/I ratio for [(14)C]ethanol and (3)H(2)O in perfused rat hindlimb.

Changes in the microdialysis outflow-to-inflow (O/I) ratio for [(14)C]ethanol and (3)H(2)O were determined in the perfused rat hindlimb after increases and decreases in nutritive flow mediated by the vasoconstrictors norepinephrine (NE) and serotonin (5-HT), respectively. Microdialysis probes (containing 10 mM [(14)C]ethanol and (3)H(2)O pumped at 1 or 2 microl/min) were inserted through the calf of the rat. Hindlimb perfusion flow rate was varied from 6 to 56 ml x min(-1) x 100 g(-1) in the presence of NE, 5-HT, or saline vehicle. The O/I ratios for both tracers were determined at each perfusion flow rate, as was perfusion pressure, oxygen uptake (a surrogate indicator of nutritive flow), and lactate release. Both tracers showed a decreased O/I ratio as hindlimb perfusion flow was increased, with [(14)C]ethanol being higher than (3)H(2)O. NE decreased the O/I ratio compared with vehicle, and 5-HT increased it for both tracers and both microdialysis flow rates. We conclude that the microdialysis O/I ratio, while able to detect changes in total flow, is also sensitive to changes in nutritive and nonnutritive flow, where the latter still extracts tracer, but less than the former.

Size-dependent effects of microspheres on vasoconstrictor-mediated change in oxygen uptake by perfused rat hindlimb.

There are two vascular flow routes in skeletal muscle that can be accessed by different vasoconstrictors acting at selective sites in the vascular tree. Thus, angiotensin II (AII) and serotonin (5-HT), which stimulate and inhibit metabolism, do so by directing flow to nutritive and nonnutritive routes, respectively. In the present study the association between vascular flow route recruitment and metabolism was assessed by embolism with microspheres of different sizes. Latex microspheres (MS) of four sizes, 5.4 (MS5), 11.8 (MS12), 23.4 (MS23), and 93.6 microm (MS94), were injected during AII- or 5-HT-mediated constriction or under basal conditions and the effects on hindlimb oxygen uptake (VO2), perfusion pressure, and venous flow rate were determined. MS5 or MS12 partially reversed 5-HT-mediated inhibition of VO2 by 39 and 55%, respectively (P < 0.05), fully reversed AII-mediated stimulation of VO2 (P < 0.05), stimulated basal VO2 (P < 0.05), and increased pressure while only marginally (<10%) decreasing venous flow. MS23 or MS94 dose-dependently increased pressure and inhibited VO2, during basal or 5HT- and AII-mediated constriction, while only marginally decreasing venous flow. In conclusion, microspheres of less than 12 microm when injected into the constant flow perfused rat hindlimb can alter metabolism by altering flow distribution between nutritive and nonnutritive routes. Larger MS (> or =24 microm) are nondiscriminating possibly because they exceed the size of vessels in which branch points to the two vascular routes are located. Overall the findings provide further evidence for two microvascular routes in muscle, one nutritive and the other nonnutritive.

Heterogeneity of laser Doppler flowmetry in perfused muscle indicative of nutritive and nonnutritive flow.

Laser Doppler flowmetry (LDF) signal responses have been compared with metabolic changes using both a surface macroprobe and randomly placed implantable microprobes in muscles of the constant-flow-perfused rat hindlimb. Changes in response to total flow and to vasoconstrictors that are known to increase (norepinephrine, NE) or decrease (serotonin, 5-HT) hindlimb oxygen uptake were assessed. The surface macroprobe (anterior end of biceps femoris) identified only one type of LDF response characterized by increased signal in response to NE and decreased signal in response to 5-HT. Implanted microprobes (tibialis, gastrocnemius, vastus, or bicep femoris) identified sites that gave three LDF responses of differing character. These responses were where the LDF signal increased with NE and decreased with 5-HT (56.7%), where the LDF signal decreased with NE and increased with 5-HT (16.5%), or where there was no net response to either vasoconstrictor (24.7%). The data are consistent with discrete regions of nutritive and nonnutritive flow in muscle where flow in each as controlled by vasoconstrictors relates directly to the metabolic behavior of the tissue.

Spatial distribution of nutritive and nonnutritive vascular routes in perfused rat hindlimb muscle using microspheres.

Skeletal muscle appears to have two vascular flow routes, nutritive and nonnutritive, where the balance of flow is controlled by vasoconstrictors. In the present study, spatial distributions of the two flow routes in muscles of the perfused rat hindlimb were attempted using fluorescent microspheres (15 microm in diameter). Microspheres were injected during steady-state perfusion with norepinephrine (proposed recruiter of nutritive flow), serotonin (proposed recruiter of nonnutritive flow), or vehicle. The three-dimensional location of individual microspheres in representative muscles was determined using a Fluorescent Imaging CryoMicrotome. Norepinephrine and serotonin each increased perfusion pressure (P < 0.05) but stimulated and inhibited oxygen consumption (P < 0.05), respectively. The distribution of microspheres lodged in muscle was independent of the agent used. Spatial perfusion indices for norepinephrine, serotonin, and vehicle did not differ from each other. Similarly, there was no difference in these indices for a theoretical distribution where microspheres were deliberately positioned in muscle bundle capillaries or interfibrillar connective tissue vessels. We conclude that the nutritive and nonnutritive flow routes are distributed throughout muscle sections consistent with their locations in muscle bundle capillaries and interfibrillar connective tissue, respectively.